The present invention relates to new polyhalogen-substituted cinnamic acids and cinnamic acid derivatives and a process for the preparation of known and new polyhalogen-substituted cinnamic acids and cinnamic acid derivatives.
Known halogen-substituted cinnamic acids and cinnamic acid derivatives are intermediates for the production of agrochemicals and pharmaceuticals (see DE-A 22 44 761, WO 95/30645, WO 94/26692, WO 94/7893, WO 94/26693 and U.S. Pat. No. 5,753,655).
2,4-Difluorocinnamic acid and its esters of the formula (II) can be prepared by reacting benzyl halides of the formula (I) with acetic anhydride or benzaldehydes of the formula (I) with malonic acid or malonic acid esters.
The following reaction equation illustrates this: 
A disadvantage here is the high reaction temperature needed, the unsatisfactory yield and the difficult accessibility of the compounds of the formula (I). Monatshefte der Chemie 90, 680 (1959) describes the reaction of 2,4-difluorobenzaldehyde with acetic anhydride at 180° C., 2,4-difluorocinnamic acid being obtained in a 77% yield.
In another route for the preparation of a halogen-substituted cinnamic acid derivative, 2,4-difluoro-bromobenzene is used as a starting material and this is reacted with acrylic acid with addition of triphenylphosphinepalladium dichloride and potassium carbonate in dimethylformamide at 145 to 150° C. in the course of 6 hours. The corresponding cinnamic acid derivative is obtained in a yield of only 54% (Russ. J. Org. Chem. 33 (4), 563-569 (1997)). The yield is still unsatisfactory here and high reaction temperatures and long reaction times are also needed.
Finally, it is known from EP-A-584 043 that compounds of the type Ar—CHRa—CHRbRc can be prepared if diazonium salts of the type AR—N2⊕ are reacted with compounds of the type CRa═CRbRc with formation of compounds of the type Ar—CHRa═CRbRc and the reaction is carried out in the presence of homogeneous palladium catalysts and with addition of 1 to 10 equivalents of base. This process is particularly suitable for the preparation of compounds in which the Ar radical is substituted by a sulfonic acid group, i.e. a strongly polar group. In addition to this restriction, it is disadvantageous that in this process large amounts of bases have to be added, which means additional costs and makes necessary a complicated work-up of the reaction mixture.
EP-A-584 264 describes a similar process to that of EP-A-584 043. However, the reaction is carried out in the additional presence of arylphosphanes, which is associated with further costs and further additional outlay.
There is thus still the need for a process for the preparation of polyhalogenated cinnamic acids and cinnamic acid derivatives in which, in a simple manner, at moderate temperatures, with short reaction times, without addition of base and without necessary addition of arylphosphanes, the desired products are accessible in a higher yield than hitherto.